INTRODUCTION: Mismatch between a proximal humerus locking plate and the contour of the bone may have consequences for attaining and maintaining proximal humerus fracture reduction. The purpose of this study was to quantify the proximal humerus locking plate-bone mismatch utilizing a large sample of human cadaveric humeri. Our secondary goal is to determine if any morphologic or demographic factors are associated with contour mismatch.

MATERIALS & METHODS: 97 cadaveric humeri, evenly distributed by gender, race, age, and laterality were selected from the Hamann-Todd human osteological collection at the Cleveland Museum of Natural History. Humerus length and head diameter were measured using digital calipers, and the neck-shaft angle was measured digitally. 3-hole and 5-hole 3.5 mm proximal humerus locking compression plates were fixed to the specimen with clay strips. A digital caliper was used to measure the gap distance from the plate to the bone through the proximal-most shaft screw hole. Mean plate-bone distances were calculated. T-tests or ANOVA were conducted to evaluate for differences in plate-bone distance. Correlation was determined using a simple linear regression.

RESULTS: Mean humeral length, humeral head diameter, and neck-shaft angle were 318.6 ± 21.9 and 42.1 ± 3.6 mm, and 132.8 ± 4.1 degrees, respectively. The mean plate-bone gap distance for the 3-hole plate and 5-hole plate was 1.5 ± 0.6 mm and 2.5 ± 0.9 mm., respectively (p=0.01). Female and right-sided humeri were shown to have significantly larger plate-bone gap distance when compared to their counterparts (p=0.01). Age and race had no effect on plate-bone gap distance. No correlation was found between plate-bone gap distance and humeral length (R2=0.01), head diameter (R2=0.01), or neck-shaft angle (R2=0.01).

CONCLUSION: This cadaveric study demonstrates that a contour mismatch exists between proximal humerus locking plates and the bone, which is associated with a longer plate, female gender, and right side. If the plate is used as a template for fracture reduction intra-operatively, this may lead to malreduction by translating the proximal fragment(s) medially by a distance equal to the plate-bone distance measured in this study (figure). Non-anatomic reduction, particularly at the medial calcar, is associated with inferior clinical outcomes in the literature. Therefore, even a minimal amount of displacement at the medial calcar may become clinically significant. Surgeons should be aware that imperfect proximal humerus locking plate fit may lead to malreduction, and should be prepared to utilize alterative techniques for fracture reduction intra-operatively.